Night sky is a commonly used term most often employed to refer to the sky as it is seen at night. The term is usually associated with astronomy, with reference to views of heavenly bodies such as stars, the Moon and planets that become visible on a clear night after the Sun has set.

The phenomenon of day and night is due to the rotation of a celestial body about its axis, creating the illusion of the sun rising and setting. Different bodies spin at very different rates, however. Some may spin much faster than Earth, while others spin extremely slowly, leading to very long days and nights. The planet Venus rotates once every 224.7 days, by far the slowest rotation period of any of the major planets. In contrast, the gas giant Jupiter's sidereal day is only 9 hours and 56 minutes. A planet may experience large temperature variations between day and night, such as Mercury, the closest planet to the sun. This is one consideration in terms of planetary habitability or the possibility of extraterrestrial life.

The disappearance of sunlight, the primary energy source for life on Earth, has dramatic impacts on the morphology, physiology and behavior of almost every organism. Some animals sleep during the night, while other nocturnal animals including moths and crickets are active during this time. The effects of day and night are not seen in the animal kingdom alone; plants have also evolved adaptations to cope best with the lack of sunlight during this time. For example, crassulacean acid metabolism is a unique type of carbon fixation which allows photosynthetic plants to store carbon dioxide in their tissues as organic acids during the night, which can then be used during the day to synthesize carbohydrates. This allows them to keep their stomata closed during the daytime, preventing transpiration of precious water.

Throughout the rest of history, night has primarily been a time of resting and sleep for humans, since little work or labor can be done in the dark. On the other hand, clandestine activities such as romance, sex, prostitution, crime and police activity flourish. As artificial lighting has improved, especially after the Industrial Revolution, night time activity has increased and become a significant part of the economy in most places. Many establishments, such as nightclubs, bars, convenience stores, fast-food restaurants, gas stations, distribution facilities, and police stations now operate 24 hours a day or stay open as late as 1 or 2 a.m. Even without artificial light, moonlight sometimes makes it possible to travel or work outdoors at night. The phrase "The night is young" refers to the period when the sun is below the horizon and not the period before midnight.

Night is often associated with danger and evil, because bandits and dangerous animals can be concealed by darkness. The belief in magic often includes the idea that magic and magicians are more powerful at night. Similarly, mythical and folkloric creatures as vampires and werewolves are thought to be more active at night. Ghosts are believed to wander around almost exclusively during night-time. In almost all cultures, there exist stories and legends warning of the dangers of night-time. In fact, the Saxons called the darkness of night the 'death mist'.
Source: From Wikipedia, the free encyclopedia